This invention relates to electrical sealing devices, for reducing electromagnetic interference.
It is often necessary to provide an enclosure for electronic equipment which shields the equipment against externally generated electromagnetic interference. Alternatively, the enclosure may be necessary to prevent signals generated by the equipment from interfering with other equipment.
It is usually necessary to provide some door or cover plate for the enclosure, so as to allow access to the equipment. However, it is difficult to ensure good, uniform electrical contact between the door or cover plate and the surrounding part of the enclosure. Accordingly, there is a tendency for signal leakage to occur.
It has been suggested to use conductive gaskets for improving the electrical sealing between a door or cover plate and the enclosure. One form of gasket consists of a multiplicity of wire elements embedded in an insulating resilient material. Examples of such gaskets are described in British Patent Specification No. 751496 and U.S. Pat. No. 2,885,459. A second form of gasket consists of metallic particles distributed throughout an insulating resilient material, as described in U.S. Pat. No. 3,583,930. When a slab of the material is compressed, the particles provide a path of relatively low conductivity between opposite faces. Both forms of the gasket have a substantial mechanical stiffness, so that a large force is required to provide the required compression of the gasket round the edges of a large door or panel. Apart from the difficulties which may be experienced by a service engineer, for example, in closing the door, or replacing the panel, the force may be sufficient to cause distortion of hinges, catches, or of the door or panel itself.
One object of the present invention is therefore to provide an electrical sealing device which requires considerably smaller compressive forces than the gaskets referred to above.
Another problem which arises with such equipment is that it is often necessary to provide a hole in the wall of the enclosure for wiring to pass through, and such a hole may allow electromagnetic interference to pass into or out of the enclosure. In order to minimize such interference, the hole is usually made of just sufficient size to accept the wiring. Sometimes the shielding braid on the wiring is held in a clamp around the periphery of the hole. This can, however, lead to difficulties in the initial fitting or replacement of the wiring, particularly if the wiring consists of one or more relatively thick and stiff cables.
Another object of the present invention is therefore to provide an electrical sealing device which can electrically seal a hole through which wiring passes, in such a way as to permit easy removal or replacement of the wiring.